It really depends on the type of ski and the terrain. It definitely is true that ski design has drastically shortened lengths, and that technique, while slower to respond, has begun to take advantage of them.
If you want a quick eye-opener of how fun a short ski can be, get a set of short slaloms in the 155-165cm range. While rough snow will be more of a workout, they do make really fun turns at respectable speeds (higher than most resorts allow). They do require a different style of skiing to be used effectively, but it's more than worth the effort. Even my GS skis have dropped to around 180cm to take advantage of the technology. A good line beats raw speed any day.
With that said, my powder/rough snow skis are still in the 190cm range, and length will always be able to bridge the rough spots and save your legs.
For a one-ski fits-all-terrain, I still would go with the 180cm range in a mid-fat or all-mountain design. If you can afford specialized tools, a shorty slalom for ripping on the groomers and a mid-fat on the off-piste stuff is a good combination.
Demo a bunch of sizes of the same ski to see which one works for you. Sometimes that's all you need to do to make up your mind.